Passive mold cooling and heating system

ABSTRACT

A mold for molding articles of plastic or other like materials, having a plurality of vertically extending internal passages, is connected by inlet and return conduits into a closed loop that includes a heat exchanger partially filled with a liquid supply of a cooling fluid or a heating fluid. For cooling the heat exchanger is positioned with its liquid level above the top of the mold, the inlet conduit connects the lower part of the heat exchanger, below the liquid level, to the lower ends of the mold passages, and the return conduit connects the upper ends of the mold passages to the upper part of the heat exchanger; for heating these relationships are reversed. In operation the fluid changes its physical state at critical points in the mold passages, going from liquid to vapor for cooling and from vapor to liquid for heating; circulation in the closed loop is in response to the effect of gravity. Operation of the heat exchanger is controlled to maintain a controlled pressure in the closed loop.

This is a division of application Ser. No. 675,877 filed Nov. 28, 1984,now U.S. Pat. No. 4,623,497.

BACKGROUND OF THE INVENTION

In the molding of articles of plastic and other like materials, as ininjection molding, blow molding, rotational molding, compressionmolding, reaction injection molding and other processes, efficientoperation of the molding process depends upon effective control ofthermal conditions in the mold. Thus, in many plastic molding processesrelatively rapid cooling of the mold to a temperature at which themolded article can be removed without distortion is a critical part ofthe process. In other plastic molding procedures, on the other hand, itis necessary to heat the mold in a rapid and controlled manner toachieve acceptable efficiency levels. If the techniques employed forcooling (or heating) of the molding apparatus are too slow, the moldingequipment must be slowed down to a speed that is economicallyundesirable. On the other hand, excessively rapid thermal changes in themold may lead to poor quality in the molded articles.

In those molding processes in which the molded article must be cooled inthe mold prior to removal, it has been conventional practice to providea series of passages for passing a cooling liquid through the mold, atleast some of the cooling passages being located quite close to the moldcavity or cavities. Chilled water or other cooling liquid circulatedthrough these mold passages cools the mold and the molded article sothat the article can be removed from the mold without distortion. Thisprocedure is inherently somewhat inefficient because a large volume ofcooling liquid must be circulated through the mold to achieve thedesired cooling. This conventional procedure is also rather difficult tocontrol so as to achieve a maximum cooling rate without cooling the moldtoo much, which may interfere with efficient operation of the mold inthe next cycle. Moreover, the conventional cooling technique isinherently non-selective; most of the mold is cooled, but it is far moredesirable to concentrate cooling in that portion of the mold immediatelyadjacent to the mold cavity. In those processes that require heating ofa mold the same basic techniques are conventionally employed, exceptthat the circulated fluid is a hot liquid. The same problems arepresented.

SUMMARY OF THE INVENTION

The present invention relates to a new and improved passive system forcooling or heating a mold used in molding articles of plastic or otherlike materials without pumping large quantities of liquid throughpassages in the mold for this purpose; the system utilizes the latentheat of vaporization (or condensation) of a heat transfer fluid forimproved efficiency and achieve circulation through passages in the moldand through the balance of the system in response to the effect ofgravity.

It is a principal object of the present invention, therefore, to providea new and improved passive mold cooling and heating system in whichefficient and selective heat transfer is obtained, within a mold for themolding of articles from plastic or like materials, by utilizing thelatent heat of vaporization or condensation of a heat transfer fluidcirculated through internal passages in the mold by gravitationaleffects, without requiring pumping of large volumes of liquid throughthe mold.

A related object of the invention is to provide a new and improvedpassive mold cooling and heating system that is simple and economicalyet highly reliable in operation and that can be controlled to optimizethe molding process.

In one specific aspect, the invention relates to a passive coolingsystem for cooling a mold for molding articles of plastic or likematerial without substantial pumping, the mold including a plurality ofvertically extending internal passages for passing a cooling fluidthrough the mold. The cooling system comprises a condenser heatexchanger partially filled with a supply of cooling fluid in a liquidstate, the cooling fluid constituting a fluid which changes from itsliquid state to its vapor state in response to temperatures occurring incritical portions of the mold passages during operation of the mold, andthe heat exchanger being positioned so that the upper level of liquid inthe heat exchanger is approximately at or above the top of the mold. Aninlet conduit connects the heat exchanger, from a point below the upperlevel of the fluid, to the lowermost ends of the mold passages, and areturn conduit connects the upper ends of the mold passages back to theheat exchanger; the heat exchanger, the mold passages, and the inlet andreturn conduits constitute a closed loop. The system further comprisescontrol means for controlling operation of the heat exchanger tomaintain a controlled pressure in the loop while condensing vaporizedfluid back to its liquid state as the fluid returns from the mold to theheat exchanger; the heat exchanger control means includes means toadjust the controlled pressure to establish a minimum limit temperaturefor cooling of the mold and means for actuating the heat exchanger tovary the loop pressure in synchronism with the molding cycle.

In another specific aspect, the invention relates to a passive heatingsystem for heating a mold for molding articles of plastic or likematerial without substantial pumping, the mold including a plurality ofvertically extending internal passages for passing a heating fluidthrough the mold. The heating system comprises an evaporator heatexchanger partially filled with a supply of heating fluid in a liquidstate, the heating fluid constituting a fluid which changes from itsvapor state to its liquid state in response to temperatures occurring incritical portions of the mold passages during operation of the mold, andthe heat exchanger being positioned so that the upper level of liquidfluid in the heat exchanger is approximately at or below the bottom ofthe mold. An inlet conduit connects the heat exchanger, from a pointabove the upper level of the fluid, to the upper ends of the moldpassages, and a return conduit connects the lower ends of the moldpassages back to the heat exchanger; the heat exchanger, the moldpassages, and the inlet and return conduits constitutes a closed loop.The system further comprises means for controlling operation of the heatexchanger to vaporize fluid from the liquid supply in the heat exchangerto maintain the conduits and the mold passages filled with vapor at acontrolled pressure; the heat exchanger control means includes means toadjust the pressure to establish a maximum limit temperature for heatingof the mold and means for actuating the heat exchanger to vary the looppressure in synchronism with the molding cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a passive mold cooling systemconstructed in accordance with one embodiment of the invention;

FIG. 2 is a graph showing pressure variations employed in one mode ofoperation of the system of FIG. 1; and

FIG. 3 is a schematic illustration of a passive mold heating systemconstructed in accordance with another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of a passive cooling system 10 forcooling a mold 11 employed to mold articles of plastic or like materialin a molding process that requires cooling of the molded article priorto its removal from mold 11. Mold 11, in the illustrated system, is ofthe general kind used in injection molding machines and similarequipment. It comprises a fixed mold half 12 and a removable mold half13, the parting line of the mold being generally indicated by thephantom line 14. The mechanism for moving mold half 13 into and out ofthe illustrated closed position, which operates in accordance with apredetermined mold timing cycle, is generally illustrated by a pistonrod 15. The arrows A show the direction of movement for mold half 13.

In the center of mold 11 there is a mold cavity 16, a part of cavity 16being formed in each of the mold halves 12 and 13. A fill passage 17 inmold 11 permits the feeding of raw plastic or like material into moldcavity 16. The fixed mold half 12 is provided with a plurality ofinternal passages 18 that extend through this half of mold 11 in avertical direction (only one passage 18 is shown). Similarly, themovable mold half 13 includes a plurality of vertically extendinginternal passages 19, only one passage 19 being illustrated. In bothmold halves, the internal passages 18 and 19 pass quite close to moldcavity 16.

It will be recognized that the illustration of mold 11 has beensubstantially simplified. Mold 11 may include a plurality of moldcavities 16, which may all be of the same shape or may be of differingconfigurations. For purposes of the present invention, passages 18 and19 must extend generally vertically through mold 11, but they need nothave the illustrated configuration; sharper bends in these passages arepermissible. The simple fill passage 17 may be much more complex inconfiguration.

Cooling system 10 further comprises a heat exchanger 20, which in thisinstance is employed as a condenser. Heat exchanger 20 includes aninternal coil 21 having an inlet 22 and an outlet 23. Coil 21 isconnected to a cooling source 24, shown as a conventional refrigerationsystem. Thus, in simplified form, cooling source 24 is shown ascomprising a compressor 25, a condenser 26, and an expansion valve 27.Expansion valve 27 is connected to the inlet 22 of coil 21 and theoutlet 23 of coil 21 is returned to compressor 25. The arrows B indicatethe direction of flow of refrigerant through the closed systemcomprising coil 21 and refrigerating apparatus 24. Other cooling systemsmay be substituted for the illustrated refrigeration apparatus ofcooling source 24.

In system 10, heat exchanger 20 is partially filled with a supply ofcooling fluid in its liquid state. The cooling fluid 30 may be water,usually with some additives to prevent rust, etc.; the use of water isassumed in the following description of system 10. On the other hand,the heat transfer fluid 30 may also constitute a conventionalrefrigerant such as the commercially available R-12, R-22, or R-114Freon refrigerants. Heat exchanger 20 should be positioned with theupper level 31 of the liquid coolant supply 30 approximately at andpreferably above the top of mold 11, for reasons to be made apparent inthe operational description of system 10. An inlet conduit 32 connectsheat exchanger 20 to the lower ends of the passages 18 and 19 throughmold 11. At heat exchanger 20, the connection for inlet conduit 32should be made at a point well below the upper level 31 of the supply ofcooling liquid 30 in the heat exchanger. A valve 33 may be interposed ininlet conduit 32 and a pump 34 may be connected in parallel with valve33.

A return conduit 35 connects the upper ends of mold passages 18 and 19back to heat exchanger 20. At the heat exchanger end, the connectionfrom return conduit 35 is made at a point above the upper level 31 ofthe cooling liquid. Mold passages 18 and 19, heat exchanger 20, inletconduit 32, and return conduit 35 constitute a closed loop; this loopshould be effectively sealed to permit control of pressure within theloop.

A vent conduit 36 connects the top of heat exchanger 20 to a vacuum pump37. Pump 37 is shown with an external connection 38 to the atmosphere,an arrangement that is quite suitable when water is used as the coolingfluid 30. The top of heat exchanger 20 is also connected to theatmosphere through another conduit 39 that includes a solenoid actuatedvalve 41.

A pressure sensor 42 is incorporated in system 10 to sense the internalpressure in the closed loop system comprising heat exchanger 20, moldpassages 18 and 19, and conduits 32 and 35. In FIG. 1 pressure sensor 42is shown connected to the top of heat exchanger 20, but it could beconnected to return conduit 35 or to some other point in the loop.Pressure sensor 42 is connected to an electrical controller 43 that iselectrically connected to pump 37. Controller 43 also has an electricalconnection to valve 41. In many systems, it is also desirable to providea third control connection from controller 43 to refrigerating apparatus24.

Before starting operation, the passive mold cooling system 10 isconnected as shown in FIG. 1 and heat exchanger 20 is partially filledwith the heat transfer fluid 30, to level 31. For present purposes it isassumed that the heat transfer liquid is water. With valve 33 open, alimited quantity of the water flows through inlet conduit 32 and up intopassages 18 and 19 in mold 11. To assure complete filling of the moldpassages and conduits 32 and 35 with the heat transfer liquid, however,valve 33 is closed and pump 34 is actuated to pump the cooling liquidthrough the closed loop as indicated by the arrows C and D. This is alsoeffective to purge the closed loop system of any entrapped air. Once theclosed loop has been purged and filled with cooling liquid 30, pump 34may be shut off and valve 33 opened.

The operation of system 10 is based on utilization of the latent heat ofvaporization of the cooling liquid 30 (water). It is also intended toprovide maximum cooling on a selective basis at critical portions of themold passages 18 and 19, the portions 18A and 19A of those passages mostclosely adjacent to mold cavity 16. For efficient operation, it isdesirable to exercise control over the flash temperature of the coolingliquid. This is accomplished by adjusting the pressure within heatexchanger 20 to a predetermined level; that pressure may then bemaintained relatively constant during operation of system 10. Assumingthat the desired pressure within the closed loop system is belowatmospheric, to afford a flash temperature below 212° F., pressuresensor 42 and controller 43 actuate pump 37 to pump air out of heatexchanger 20 until the desired pressure is attained. For any overshootbelow the desired pressure, pressure sensor 42 and controller 43 reactto shut off pump 37 and open valve 41 until the desired pressure levelis reached.

Thereafter, in general, there may be no pumping of cooling liquid in theoperation of system 10. When mold 11 is closed by operation of piston 15and heated plastic is injected into mold cavity 16 through fill passage17, the walls of the mold cavity are heated and this heat is conductedto the closely adjacent critical portions 18A and 19A of the moldcooling passages. The hot critical portions 18A and 19A of the moldpassages vaporize the water. This produces rapid cooling at the hottestpoints in mold 11 without undue cooling of other portions of the mold.Heat absorption is highly efficient, due to the utilization of thelatent heat of vaporization of the cooling fluid. The vaporized coolingfluid (steam) rises through passages 18 and 19 and passes through returnconduit 35 to bubble up through the liquid coolant supply 30 in heatexchanger 20. The refrigerating apparatus 24 circulates refrigerantthrough coil 21, continuously condensing the vaporized cooling fluid 30back to its liquid state.

During operation of system 10, the pressure and temperature of thecooling liquid 30 are continuously regulated; in some systems, pressurecontrol may be adequate. Thus, the control comprising pressure sensor 42and controller 43 may actuate pump 37 and valve 41 to maintain thepressure within heat exchanger 20, and in the complete closed loop ofwhich the heat exchanger 20 is a part, at a constant level. On the otherhand, a similar control may be exercised by regulation of the operationof refrigerating apparatus 24, controlling the rate of cooling forcondenser 20. In some systems, it may be desirable to employ bothcontrol techniques conjointly.

Completely passive operation of system 10, as described, may be highlyeffective and efficient. In some instances, however, the rate ofmovement of vaporized cooling fluid upwardly through passages 18 and 19and conduit 35 due to the effect of gravity may be inadequate so thatthere is a tendency for vapor bubbles to accumulate in the mold passagesIn these circumstances, it may be desirable to operate pump 34 at a lowlevel to provide a slow circulation of liquid through the closed loop asindicated by arrows C and D, thereby assuring effective transfer of heatfrom the critical portions 18A and 19A of the mold passages to condenser20.

In system 10, virtually no cooling action takes place below thevaporization temperature of the cooling fluid 30. Thus, if the coolingfluid is water and the pressure in the system is nominal atmospheric,system 10 will not cool mold 11 below 212° F. This is of substantialadvantage in preventing undue cooling of the mold, a common problem inconventional cooling systems. The cooling effect is quite selective,with the vaporization of the cooling liquid taking place almost entirelyin those portions 18A and 19A of the mold passages that are immediatelyadjacent to mold cavity 16. This materially reduces the inefficiencyassociated with excessive transfer of heat from other portions of mold11 to condenser 20.

In the foregoing description of the operation of system 10 (FIG. 1) itis assumed that the pressure in the closed loop formed by elements 20,32, 18 and 19, and 35 is held constant. For some systems, that mode ofoperation may not produce optimum results. For further improvement inthe operation and to obtain high quality molded articles with a minimummolding cycle, in at least some systems it may be desirable to reducethe pressure, cyclically, in the closed loop during the critical portionof the molding cycle when the mold has closed and hot plastic isintroduced into the mold, in order to accelerate vaporization of theheat-transfer fluid and cooling of the critical portions 18A and 19A ofthe mold passages.

This mode of operation can best be understood by reference to FIG. 2, inwhich loop pressure is plotted as a function of molding cycle time. InFIG. 2, point X in each mold cycle indicates the time that the mold isclosed and filling of the mold cavity with hot plastic commences. PointY in each mold cycle represents the time that the mold begins to open.

Shortly before time X in each mold cycle (FIG. 2), controller 43actuates pump 37 to effect a substantial reduction in the pressure inheat exchanger 20, and hence in the closed loop including the moldcooling passages 18 and 19. This initiates vaporization within the mold(during operation, the mold is hotter than any other part of the closedloop system). This produces an appreciable temperature differentialbetween the critical mold passage portions 18A, 19A and the mold cavity16 immediately prior to filling of the mold. Shortly before the moldopens (point Y) the pressure is restored to its original level. In thismanner, the heat transfer process is synchronized with the moldingoperation for optimum results, while avoiding undue cooling during thoseportions of the mold cycle when the mold is open.

FIG. 3 affords a schematic illustration of a passive heating system 110that utilizes the same basic operational principles as the passivecooling system 10 of FIG. 1. As before, the passive heating system 110is utilized for thermal control of a mold 11 used for molding articlesof plastic or like material, mold 11 including fixed and movable moldhalves 12 and 13 affording one or more mold cavities 16 into whichplastic is fed through appropriate passages 17, the mold actuationmechanism being indicated only by the piston rod 15 for mold half 13.Again, the mold is provided with multiple internal passages 18 and 19;in this instance the passages are utilized to heat the mold.

A heat exchanger 120 is included in system 110, FIG. 3, and includes aninternal coil 121 having an inlet 122 and an outlet 123. The outlet 123is connected to a heater 125 in a conventional heating apparatus 124that also includes a pump 126 connected back to the inlet 122 of coil121. Heat exchanger 130 functions as an evaporator in system 110.Accordingly, the closed system comprising heating apparatus 124circulates hot water or other heating fluid through coil 121 inevaporator 120.

The controls for heat exchanger 120 may be the same as in the previouslydescribed system, comprising a vent conduit 36 connected to a pump 37.In this instance the outlet of pump 37 is connected to a storage vesselor accumulator 138 rather than to the atmosphere. Accumulator 138 isconnected to a solenoid actuated valve 41 in an inlet conduit 39returned to the interior of evaporator 120. Pressure control is providedby a pressure sensor 42 and controller 43 with the controller 43actuating pump 37 and valve 41. Controller 43 may also be used tocontrol operation of heating apparatus 124. Evaporator 120 is partiallyfilled with a heating fluid 130, in its liquid state, to an upper level131. Fluid 130 may be water, a commercial refrigerant, or other fluid.

An inlet conduit 132 connects heat exchanger 120, from a pointsubstantially above the liquid level 131, to the upper ends of thevertically extending mold passages 18 and 19 in mold 11. A valve 133 maybe interposed in the inlet conduit 132 with a small pump 134 in parallelwith the valve. A return conduit 135 connects the lower ends of moldpassages 18 and 19 back to heat exchanger 120. Heat exchanger 120,conduits 132 and 135, and mold passages 18 and 19 afford a closed loop,as in the previous embodiment.

When system 110 is placed in operation, hot fluid from heating apparatus124, passing through coil 121, heats the heat transfer fluid 130 inevaporator 120, vaporizing an appreciable portion of the heat transferfluid. Pump 134 may be utilized at the outset to assure purging ofconduits 132 and 135 and mold passages 18 and 19. In normal operationpump 134 is shut off and valve 133 is open. The control comprising pump37, valve 41, sensor 42 and controller 43 establishes a predeterminedpressure in the system, as before. Accumulator 138 merely serves asstorage for excess heat transfer fluid 130.

System 110 functions on the same basis as system 10 except that it isemployed for heating of mold 11 instead of cooling of the mold, with theheating effect based upon condensation of fluid 130 from its vapor stateto its liquid state. The closed loop comprising the mold passages 18 and19 is filled with hot heat transfer fluid 130 in its vapor state at apredetermined pressure. At any portion of the mold passages 18 and 19below the condensation point for the gaseous heat transfer fluid,condensation occurs. Most condensation occurs at the critical portions18A and 19A of the internal mold passages. The condensed heat transferfluid drains, by the effect of gravity, back through return conduit 135into exchanger 120. There is no need for active pumping of hot gasthrough the closed loop comprising the evaporator and the mold passages;by maintaining the system at a substantially constant pressure, the hotgaseous heat transfer fluid quickly refills the mold passages, replacingthe gas that has been condensed. Thus, in system 110 as in system 10 thebasic circulation is that afforded by the effect of gravity based upon achange of physical state of the heat transfer fluid within the passagesof mold 11 used for cooling or heating purposes.

For system 110, as for system 10, it may be desirable to vary the closedloop pressure synchronously with the molding cycle. That is, controller43 may be programmed to increase the pressure in the loop 120, 132, 18,19, and 135 shortly prior to filling of cavity 16 in mold 11, with adecrease to an initial pressure level to coincide approximately withopening of the mold. The pressure/time characteristic would thus be thereverse of that shown in FIG. 2. In both cases, of course, a timinginput (not shown) to controller 43 may be necessary from the mechanismactuating the mold.

From the foregoing description of FIGS. 1 and 3, it is apparent that thesystem embodiments of the invention all function in accordance with thesame basic method, regardless of whether the mold is to be heated orcooled. Thus, for each temperature modification arrangement, theinternal passages of the mold 11 are connected into a closed loop with aheat exchanger, the relative elevations of the mold in the heatexchanger being determined so that the heat transfer fluid tends to flowfrom the mold to the heat exchanger in response to the effect of gravitywhen the fluid is in a given physical state (vapor state for cooling,liquid state for heating). That closed loop is filled with a heattransfer fluid that changes from an initial physical state to the givenphysical state required for return to the heat exchanger in response tothermal conditions occurring in the critical portions of the moldpassages; in both instances a reservoir of liquid heat transfer fluid ismaintained in the heat exchanger. The heat exchanger itself iscontrolled to maintain a controlled pressure in the closed loop and tochange the fluid back to the initial physical state required for fillingthe closed loop at approximately the same rate as the fluid returns fromthe mold to the heat exchanger. This operational control may be effectedby pump 37 and valve 41, by control of the input to the heat exchangercoil, or by a combination of the two. The loop pressure may be heldconstant, or it may be varied in synchronism with the molding cycle.

I claim:
 1. A passive cooling system for cooling a mold for moldingarticles of plastic material without substantial pumping, the moldincluding a plurality of vertically extending internal passages forpassing a cooling fluid through the mold, the system comprising:acondenser heat exchanger partially filled with a supply of cooling fluidin a liquid state, the cooling fluid constituting a fluid which changesfrom its liquid state to its vapor state in response to temperaturesoccurring in critical portions of the mold passages during operation ofthe mold, the heat exchanger being positioned so that the upper level ofliquid fluid in the heat exchanger is approximately at or above the tipof the mold; an inlet conduit connecting the heat exchanger, from apoint below the upper level of the fluid, to the lower ends of the moldpassages; a return conduit connecting the upper ends of the moldpassages back to the heat exchanger; the heat exchanger, the moldpassages, and the inlet and return conduits constituting a closed loop;and heat exchanger control means for controlling operation of the heatexchanger to maintain a controlled pressure in the loop while condensingvaporized fluid back to its liquid state as the fluid returns from themold to the heat exchanger; the heat exchanger control means includingmeans to adjust the controlled pressure to establish a minimum limittemperature for cooling of the mold and means for actuating the heatexchanger to vary the loop pressure in synchronism with the moldingcycle.
 2. A passive mold cooling system according to claim 1 in whichthe cooling fluid is water.
 3. A passive mold cooling system accordingto claim 1 in which the cooling fluid is a Freon refrigerant.
 4. Apassive mold cooling system according to claim 1 in which one of theinlet and return conduits includes pump means for circulating thecooling fluid through the closed loop at a slow rate, insufficient toafford substantial cooling of the mold, to assure movement of vaporizedcooling fluid back to the heat exchanger.
 5. A passive mold coolingsystem according to claim 1 in which the heat exchanger control meansfurther comprises:pressure sensing means for sensing the pressure in theclosed loop; first and second inlet/outlet conduits for the heatexchanger, one including a control pump and the other a control valve;and controller means, connected to the pressure sensing means, foractuating the control pump and the control valve in response tovariations in pressure in the closed loop.
 6. A passive mold coolingsystem according to claim 1 in which the heat exchanger includes aninternal coil and the heat exchanger control means furthercomprises:pressure sensing means for sensing the pressure in the closedloop; chiller apparatus for circulating a chilled heat transfer fluidthrough the heat exchanger coil; and controller means, connected to thepressure sensing means, for adjusting operation of the chiller apparatusin response to variations in pressure in the closed loop to maintain aconstant pressure.
 7. A passive heating system for heating a mold formolding articles of plastic material without substantial pumping, themold including a plurality of vertically extending internal passages forpassing a heating fluid through the mold, the system comprising:anevaporator heat exchanger partially filled with a supply of heatingfluid in a liquid state, the heating fluid constituting a fluid whichchanges from its vapor state to its liquid state in response totemperatures occurring in critical portions of the mold passages duringoperation of the mold, the heat exchanger being positioned so that theupper level of liquid fluid in the heat exchanger is approximately at orbelow the bottom of the mold; an inlet conduit connecting the heatexchanger, from a point above the upper level of the fluid, to the upperends of the mold passages, a return conduit connecting the lower ends ofthe mold passages back to the heat exchanger; the heat exchanger, themold passages, and the inlet and return conduits constituting a closedloop; and heat exchanger control means for controlling operation of theheat exchanger to vaporize fluid from the liquid supply in the heatexchanger to maintain the conduits and the mold passages filled withfluid vapor at a controlled pressure; the heat exchanger control meansincluding means to adjust the pressure to establish a maximum limittemperature for heating of the mold and means for actuating the heatexchanger to vary the loop pressure in synchronism with the moldingcycle.
 8. A passive mold heating system according to claim 7 in whichthe heating fluid is water.
 9. A passive mold heating system accordingto claim 7 in which the heating fluid is a Freon refrigerant.
 10. Apassive mold heating system according to claim 7 in which the heatexchanger control means further comprises:pressure sensing means forsensing the pressure in the closed loop; first and second inlet/outletconduits for the heat exchanger, one including a control pump and theother a control valve; and controller means, connected to the pressuresensing means, for actuating the control pump and the control valve inresponse to variations in pressure in the closed loop to maintain aconstant pressure.
 11. A passive mold heating system according to claim7 in which the heat exchanger includes an internal coil and the heatexchanger control means further comprises:pressure sensing means forsensing the pressure in the closed loop; heater apparatus forcirculating a heated heat transfer fluid through the heat exchangercoil; and controller means, connected to the pressure sensing means, foradjusting operation of the heater apparatus in response to variations inpressure in the closed loop to maintain a constant pressure.